Although commercial application of ultrasonic holography as been accurately pursued by many persons in the scientific and industrial communities for many years, only limited results have been obtained even though it was once thought that ultrasonic holography held great promise. It was felt that the application of ultrasonic holography was particularly applicable to the fields of nondestructive testing of materials and medical diagnostics of soft tissues that are relatively transparent to ultrasonic radiation. One of the principal problems that has been encountered and not effectively resolved is the difficulty of obtaining quality and consistent images.
Solutions to this problem have been elusive, in part because of the difficulty in identifying the many causes that contribute to the problem. It is believed that one of the major problems has been the difficulty in devising or constructing accurate an optical reconstruction system that is compatible with an ultrasonic hologram imaging apparatus to provide consistent, high quality images.
FIG. 1 shows a typical prior art "real time" ultrasonic holographic imaging apparatus generally designated with the numeral 10. The apparatus 10 is intended to ultrasonically inspect the interior of an object 12 such as the soft tissue of a human limb. The ultrasonic holographic imaging apparatus 10 generally has a hologram generating system 14 for generating an ultrasonic hologram. The apparatus 10 also includes a prior art hologram viewing subsystem (optical-subsystem) 16 for optically viewing the interior of the object 12 from a first order diffraction from the formed ultrasonic hologram.
The system 14 includes an object ultrasonic transducer 18 for generating plane waves through a coupling medium 20 contained in a deformable membrane 22. The deformable membrane 22 intimately contacts the object 12 on one side and a deformable membrane 24 contacts the object on the other side to provide ultrasonic coupling with minimum energy loss or wave distortion. The deformable membrane 24 forms part of the side wall of a container 28 that contains a liquid coupling medium 30.
One of the components is a prior art horizontal ultrasonic imaging lens system 32 for viewing a large field and focusing at a desired object focal plane 34. The ultrasonic imaging lens system 32 focuses the ultrasonic energy onto a hologram detector surface 36. The ultrasonic imaging lens system 32 includes a large diameter object lens 38 that is moveable with respect to a large diameter lens 40 for adjusting the desired focal plane 34 in the object 12. The ultrasonic imaging lens system 32 includes a mirror 41 for reflecting the ultrasonic energy vertically approximately 90.degree. and onto the hologram detection surface 36 to form the hologram.
An ultrasonic reference transducer 42 directs coherent ultrasonic plane waves through the liquid medium 30 at an off-axis angle to the hologram detector surface 36 to form the hologram. Preferably, the hologram detection surface 36 is a liquid surface of a liquid/gas interface that is supported in an isolating dish or mini-tank 44.
The prior art hologram viewing system 16 includes an optical lens 45 to achieve an effective point source of a horizontal coherent light beam from a laser (not shown). The focused coherent light beam is reflected from a mirror 46, 90 degrees, through a collimating optical lens 47 and then onto the hologram detector surface 36 to illuminate the hologram and generate diffracted optical beams. The diffracted coherent light radiation containing holographic information is directed back through the collimating lens 47 and separated into precisely defined different order diffracted beams in the focal plane of the collimating lens 47. A filter 48 is used to block all but first order diffracted beams from a viewing lens 49 to enable a human eye, a photographic film or a video camera to record, in "real time", an image of the object taken at the object focal plane. As previously mentioned, although such an apparatus is operable, it has been difficult to obtain quality and consistent images.
One of the principal objects and advantages of this invention is to provide an improved ultrasonic holographic imaging apparatus that has a unique optical reconstruction system that overcomes many of the disadvantages of the previous optical reconstruction systems to provide images of high quality.
These and other objects and advantages of this invention will become apparent upon reading the following detailed description of a preferred embodiment.